The present invention relates to a process for surface treatment of a stainless steel sheet (including a stainless steel strip) according to which electro-chemical treatment is performed using the stainless steel sheet as a cathode for forming a noncorroding film on the surface thereof.
Chrome plating is known as a process for surface treatment of this type. Chrome plating is a kind of electroplating according to which a surface film of pure chrome is formed on the surface of the stainless steel sheet by electrodeposition of metal chrome. This surface film is advantageous in that it is noncorroding and has excellent gloss. This film is utilized for treating stainless steel sheets which are inexpensive but are not sufficiently noncorroding, such as sheets according to JIS SUS430.
However, this chrome plating adopts as a treatment solution a mixture of sulfuric acid and chromic acid of high concentration. For this reason, the waste solution from the treatment contains a large amount of chromic acid and thus requires costly treatment for prevention of pollution.
Chrome plating further requires a large current of over several amperes per dm.sup.2 and a long treatment time. This plating process thus results in great power consumption (e.g., integrated current density 6,000 A.sec/dm.sup.2) and higher treatment cost.
In view of these problems, it has been proposed, as an alternative to chrome plating, to perform an electrochemical treatment using a mixture of chromic acid, dichromic acid and phosphoric acid as a treatment solution, and a stainless steel sheet as a cathode for forming a noncorroding film on the surface of the stainless steel sheet.
This process is advantageous in that the concentration of residual chromic acid in the solution is low, and the treatment of the waste solution is easy. Furthermore, a corrosion resistance comparable to that obtained with chromic plating may be obtained with a smaller current density and a shorter conduction time.
However, this process has drawbacks in that resistance to sulfurous acid which is essential in stainless steel sheets for automobiles is poor, and surface gloss is poor, thus reducing the product value as compared with chrome-plated stainless steel sheets.